1. Field of the Invention
The present invention relates to an output driver circuit in semiconductor device.
2. Description of the Related Art
Larger scale semiconductor integrated circuits and more microminiaturized circuit elements result in a lower power-supply voltage and a larger number of I/O pins, which necessitates a plurality of power-supply voltages in an apparatus in which a plurality of semiconductor devices are connected.
FIG. 4 shows a prior art output driver circuit and its periphery.
A semiconductor device 10 and a semiconductor device 20 are connected with each other by a bi-directional bus line 30. In the semiconductor device 10, an output driver circuit 12, which operates on output signals S1 and S2 from a pre-driver circuit 11, is formed at its output stage. In the output driver circuit 12, the source S and the drain D of a pMOS transistor Qu are respectively connected with a wiring at a power-supply potential VDD and an output OUT, and the source S and the drain D of an nMOS transistor Qd are respectively connected to a wiring at a reference potential VSS and the output OUT. For instance, the semiconductor device 10 operates at 3.3 V, whereas the semiconductor device 20 operates at 5.0 V, with VDD=3.3 V and VSS=0 V.
When a signal is output from the semiconductor device 20 to the bi-directional bus line 30, the signals S1 and S2 are respectively set to a high and a low in order to set the output from the output driver circuit 20 at a high impedance state.
However, when the output OUT is at 5 V, a forward current flows at the PN junction diode between the drain D and an N-well of the pMOS transistor Qu, and the current flows to a wiring of the power-supply potential VDD through the pMOS transistor Qu to increase the potential at VDD to a near 5 V. This may cause an erroneous operation in a circuit connected to the power-supply potential VDD, e.g., an input buffer circuit (not shown) in particular, or may accelerate the process of degradation due to a high level of voltage stress to reduce reliability.
If, in the semiconductor device 10, the gate oxide film of the MOS transistors connected to the bi-directional bus line 30 is made thicker than that of another, the number of manufacturing steps increases, resulting in higher production costs.
If 5 V is used as the power-supply potential VDD to prevent the current from flowing in the reverse direction at the pMOS transistor Qu, it becomes necessary to provide an interface circuit between the pre-driver circuit 11 and the output driver circuit 12. Furthermore, if 5 V is supplied from the outside of the semiconductor device 10, it will restrict the number of pins used for signal input/output at the semiconductor device 10 and, therefore, will conflict with the need for a larger number of pins. If, on the other hand, a step-up circuit is provided for the output driver circuit 12 in the semiconductor device 10 in order to satisfy the need for more pins, the area occupied by the step-up circuit will be relatively large since the drive capacity of the circuit 12 is relatively large, thereby preventing higher integration of the circuits in the semiconductor device 10.